Polycarbonate resins are used in a variety of engineering plastic applications because of their excellent impact resistance, self-extinguishing properties, dimensional stability and high heat resistance compared to other engineering plastics.
However, the use of polycarbonate resins is limited in many applications because polycarbonates can have high notch sensibility and thus can exhibit reduced impact strength along parts of molded products that are subjected to stress. Moreover, polycarbonate resins require high molding temperatures due to the low fluidity of polycarbonates, unlike other general thermoplastic resins. Therefore, physical properties of the polycarbonate resin can be reduced by thermal decomposition due to over-heating. Also high injection pressures and velocities may be used during injection molding to compensate for the low fluidity of the polycarbonate resin, which can add further stress to parts of the molded articles. This residual stress can significantly reduce impact resistance, which can further limit the uses or applications for such resins.
Many efforts have been made to solve these problems. For example, one method to reduce notch sensibility of polycarbonate adds an impact modifier or reinforcing particles. The impact modifier can have, for example, a core/shell structure in which vinyl monomers form a shell structure and polybutadiene or acrylate rubber form a core structure. Alternatively, the impact modifier can be an olefin copolymer.
The addition of an impact modifier can significantly prevent the reduction of the impact resistance of polycarbonate resin, although residual stress can remain. However, simply adding only an impact modifier does not improve the fluidity of polycarbonate resin. Further, the polycarbonate resin can exhibit discoloration and/or reduced physical properties due to changes in the impact modifier that can result from exposure to high temperatures during processing. Therefore, uses for polycarbonate resin including only an impact modifier without improving fluidity are limited.
One method for overcoming weak points in polycarbonate resins adds acrylonitrile-butadiene-styrene (ABS) graft copolymer prepared by emulsion graft polymerization as an impact modifier and styrene-acrylonitrile copolymer (SAN) to improve fluidity. The product can be manufactured at lower processing temperatures than polycarbonate resin, because the thermoplastic resin composition prepared by mixing ABS resin and SAN resin with polycarbonate resin exhibits improved fluidity as well as reduced notch sensibility. However, molded products can exhibit significantly reduced mechanical strength of weld parts during injection molding due to limited compatibility of polycarbonate resin and SAN resin when ABS resin and SAN resin are simply mixed with polycarbonate resin.
U.S. Pat. No. 3,988,389 discloses increasing the amount of butadiene rubber of a polycarbonate, ABA resin, or SAN resin composition to improve weld strength. However, fluidity and heat stability are reduced when the amount of butadiene rubber increases.
U.S. Pat. Nos. 5,128,409 and 5,292,786 disclose mixing polyalkylmethacrylate with a composition including ABS graft copolymer and polycarbonate. This does not, however, solve the problems of decreased fluidity of the resin composition and reduced color uniformity and staining.